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MG MGB Technical - Relay next to starter relay ?

Hi, I've just rebuilt the engine in our 77 Mgbgt 1800 , initially had no ignition lights or ignition controlled circuits but the starter worked and having hot wired it and got it started noticed the lucar connectors on the 2 relays next to the fuse box were corroded and loose. I replaced them and cleaned the connections and now have power to all but when I turn the key the starter doesn't work. One relay has black, brown ,white/brown and two white/brown on the fourth spade. The other relay has black, two brown, two white/red, white/red though there is a possibility the wires may have been mixed up ....
One relay is the starter relay but what's the other one ?
I got a new relay from Mgbhive and swapped it for one, then the other but no difference. I've got 3 wiring diagrams, none show the same colour of wiring as I have and I'm at a loss now ... Anybody know if the wires are on the correct spades or what I do next ?
Brian
B D Telfer

Brian - Go to Advance Auto Wire at: http://www.advanceautowire.com/ Click on Stock Schematics, then on MGB and scroll down to the print that matches your car to see a colorized schematic for your car.
Cheers,
David DuBois

The first relay is the ignition relay, which only 77 and later cars had.

The second is the starter relay which cars from 1970 had.

The two are independant of each other.

When you turn the ignition on and off the ignition relay should click. When you turn the key to crank the starter relay should click, however that will be drowned out by the solenoid going 'clonk', and that will be drowned out by the starter turning the engine over. If the engine doesn't turn over then listen for the clonk or the click, then you will know which bit of the circuit to investigate.

If the relay doesn't click then the problem is from the ignition switch to the relay. If that clicks but there's no clonk then the problem is from the relay to the solenoid. If there is a clonk but not cranking the problem lies in the starter motor.

If neither of the relays click then it's probably the ignition switch or the connections to it.

If you come back and say which noises you are and aren't hearing you can get some more detailed diagnosis info.
Paul Hunt

Brian,
See attached sketch of relay wiring, as per my 1978 model BGT:


Brian Shaw

Thank you , all three of you ! None of the wiring diagrams show the second relay though I thought it had to be ignition related. I'm having a night off,will check it out tomorrow . Brian

B D Telfer

"None of the wiring diagrams show the second relay"

What diagrams are you looking at? My Haynes, later versions of the factory Workshop Manual/Bentley and Advance Autowire all show them.
Paul Hunt

On the Advance Autowire diagrams that I downloaded, only the starter relay shows up on the ones labeled 1977, but try the last one that is titled "Late UK Market MGB from Haynes" and see if that looks like your car. That one does have both relays.

Charley
C R Huff

This thread was discussed between 09/05/2014 and 11/05/2014

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